Vietnam's criminal probe into a national exam fraud was expanded to the third province with two more officials arrested on Friday.

Nguyen Khac Tuan, an official at the Testing Unit under Hoa Binh's Education Department, and Do Manh Tuan, vice principal of a boarding school for ethnic minorities in Lac Thuy District, were arrested and placed under criminal investigation for "abuse of power or position in performance of official duties."

Nguyen Khac Tuan, an official at the Testing Unit under Hoa Binh Province's education department, and Do Manh Tuan, vice principal of a local boarding school, were arrested on Friday for probe into a national high school exam fraud. Photo courtesy of the police

Police have accused both men of being involved in a national high school exam fraud in the province, in which the answer sheets of some candidates were altered to raise their marks.

According to the director of Hoa Binh's education department, the number of answer sheets that had been tampered with has yet to be determined. He however asserted that his department was not attempting to cover up the incident.

After the Ministry of Education and Training released the official test results of all candidates nationwide on July 11, Hoa Binh was among a number of provinces that had the public question their candidates' results.

Out of the province's over 8,900 candidates, 24 scored at least 9 on a scale of 10 in Maths, making up 4.7 percent of all candidates with such high scores in the country.

Hoa Binh's education department then launched an inspection into its exam grading process but reported no violation, prompting the education ministry to request a police investigation on July 24.

Police in Hoa Binh check the room of Do Manh Tuan, a local vice principal, for investigation in a nationan high school exam fraud. Photo by VnExpress/Duong Tam

An initial investigation soon discovered signs that a number of candidates' multiple-choice answer sheets had been altered to improve their results. Hoa Binh's education department last Saturday also reported discovering that the exam marking time did not match with the time the computer used for grading was being used.

Five officials responsible for grading the multiple-choice exams were subsequently summonned for questioning, and a criminal investigation was officially launched on Friday.

Hoa Binh is the third province caught in a criminal probe into the high school exam fraud this year, besides the northern Ha Giang and Son La Provinces.

The investigation in Ha Giang has so far led to the arrests of two senior education officials, who were found to have heavily altered the results of over 330 test sheets belonging to 114 candidates to increase their scores.

In Son La, five officials, including the provincial education department's deputy director, have been placed under criminal investigation for raising the marks for some candidates.